[Endoperez]

There are many spells that are high in the research list not because of their power, but because they don't need high levels of magic. Check Frozen Heart, Nether Darts, Will of the Fates... For their effect, the casterlevel needed seems low.

You should probably have a list of all boosters. There is some kind of a file that sums them up, but I don't remember who made it or its name. Try Arryn's site. Fire, Air and Blood have boosters only after level 3, or Fire 1 Death 1 with fire and Skull of Fire. Both Air and Blood have two boosters available after that. Earth, Nature and Death have booster available at magic 2, Nature also has N3S1 item and N4 staff that gives 2-level bonus. Death has D3 or D4 helmet, and some very nice artifacts. Water has Water Bracelet and, if you really need to reach high level and cannot summon Sea King's Court (Sea King has Water 3), Robe of the Sea.

There are few items that boost many paths at once. Staves of Elemental Mastery, Rings of Wizardy, Rings of Sorcery... and Robe of the Magi. However, the last one has nasty side-effect of imposing Horror Mark on its wielder. I think it is of stronger kind than that imposed by the spell.